1. Field of the Invention
The present invention relates to a calibration method for maintaining a print density characteristics constant in an image forming apparatus.
2. Related Background Art
In color printers, copying machines and print apparatuses for printing characters and/or images on a print medium such as a paper, in general, it is known that print output characteristics such as gradation, density and the like in a print result may be varied with change in print environments and/or time-lapse change of the apparatus itself.
The print environments may, for example, include temperature and humidity of an atmosphere into which the apparatus is installed, and, if such print environment is changed, an amount of toner adhered to the print medium may be changed in electrophotographic systems or an ink discharged amount may be changed in ink jet systems, with the result that desired output characteristics may not be obtained. Further, when a printing operation is continued for a relatively long term, a temperature within the apparatus may be changed, thereby changing the output characteristics. Furthermore, the output characteristics may be varied with a toner remaining amount.
On the other hand, in the time-lapse change, while depending upon frequency of use of the print apparatus, when the apparatus has been used for a relatively long term, characteristics of various parts of the apparatus such as a charging property of a photosensitive drum are changed, with the result that the output characteristics are changed.
In a case where a plurality of print apparatuses are used via a network as is in information processing systems, the change in the print characteristics of various print apparatuses causes another problem. That is to say, if the output characteristics are changed due to the above-mentioned factors, difference in output characteristics between the plural print apparatuses may occur. In such a case, not only the desired print characteristics cannot be obtained in the respective print apparatuses, but also, if a different printer is selected in the system, the print result will also be differentiated.
To solve the above-mentioned problems regarding the print output characteristics, generally, it is known to perform calibration. There are two general methods for performing the calibration. In one method, the calibration is effected in such a manner that a predetermined patch pattern (measuring pattern image) is outputted from the print apparatus to be calibrated and is read by a scanner or the like and calibration data is created on the basis of a read result. In the other method, the calibration is effected individually in the print apparatus. For example, the calibration is effected in such a manner that a patch pattern is formed on a photosensitive drum at a predetermined timing of the apparatus and density of the patch pattern is read by a sensor disposed in the vicinity of the drum and calibration data is created on the basis of density data. Incidentally, in the above-mentioned two methods, more specifically, the creation of the calibration data is effected to create data for renewing or updating contents of a gamma correction table in image processing.
As the former calibration method, the Inventors have proposed calibration effected between host computers constituting a system and a color printer. This calibration utilizes an operation of the user. More specifically, the patch pattern is printed out from the color printer on the basis of instruction from a server computer among plural computers constituting the system and the patch pattern is read by a scanner. In the server computer, the calibration data is created on the basis of the read scan data and the created calibration data is downloaded to the color printer. And, in the color printer, for example, gamma correction is effected by using a gamma correction table updated by the downloaded calibration data and the printing is performed on the basis of the data.
As a result, dispersion in print output characteristics between the plural print apparatuses can be reduced. Thus, for example, difference in output density characteristics between the respective print apparatuses can be eliminated to determine output density characteristic common to the plural print apparatuses, thereby stabilizing absolute density. Incidentally, the above-mentioned calibration is refereed to as “software calibration” hereinafter.
On the other hand, as the latter calibration method, the Inventors have proposed the following technique.
That is to say, in a printer engine of the print apparatus, after maximum output densities of cyan (C), magenta (M), yellow (Y) and black (K) colors are corrected at predetermined timings, engine characteristics information is obtained.
Then, a printer controller creates the calibration data on the basis of the engine characteristics information transferred from the printer engine. Then, the printer engine can perform the printing on the basis of print data corrected by using the calibration data updated by such production. According to such calibration data, particularly, change in relatively short time print output characteristics (i.e., change in output characteristics dues to change in temperature, humidity and the like) which may occur in the print apparatus can be suppressed.
The calibration of this type serves to eliminate change in output density characteristics caused in the respective print apparatuses, thereby stabilizing relative density other than the above-mentioned absolute density. Incidentally, in the following description, such calibration is also referred to as “device calibration”.
By the way, the above-mentioned two calibration types can function individually, and, if there is no correlation between them, the following problem will arise. For example, in accordance with the user's instruction, even when the software calibration is effected at a certain timing, since the device calibration is generated at the predetermined timing individually determined in the print apparatus, the output characteristics of the printer engine are changed due to the device calibration, with the result that the effect of the software calibration is not maintained. In this case, particularly, the print result having the stable absolute density cannot be obtained. Thus, in order to obtain such print result, the user must perform the software calibration frequently.
Thus, as disclosed in U.S. Patent Application No. 923992 according to the Applicant, it is considered that execution of the software calibration is correlated with execution of the device calibration and data for correcting the result of the software calibration in accordance with the device calibration is acquired and the result of the software calibration is finely adjusted on the basis of the correction data. With this arrangement, particularly, the output characteristics according to the absolute density obtained by the software calibration can be maintained and the change in the relatively short time relative density in the print apparatuses can also be eliminated.
However, in the arrangement in which the software calibration is correlated with the device calibration, if image processing conditions for effecting the respective calibrations (for example, binarizing or multi-value obtaining) are different, the fine adjustment may not be performed with high accuracy. For example, in a case where a dither method is used as the binarizing, when the software calibration is performed, a dither pattern of dot concentration type attaching importance to gradation is set, and, on the other hand, when the device calibration is performed, in a case where the printer engine outputs a patch on the basis of patch data comprised of a dither pattern of dot dispersion type attaching importance to a resolving power, in the respective calibrations, gradation characteristics obtained from the patch patterns outputted with respect to the same gradation value data may be differentiated. In such a case, it cannot be said that the calibration data obtained by the software calibration is well matched to the correction data of the calibration data obtained by the device calibration, with the result that the calibration data accurately reflecting the output characteristics of the print apparatuses cannot be obtained.
As mentioned above, the fact that the set image processing conditions as is in the dither method are differentiated between the software calibration and the device calibration is given from the following arrangement. That is to say, there is an arrangement in which, in a printer driver of the host computer, the user can set a binarizing method as the image processing conditions in accordance with an image to be printed directly or indirectly (through selection of the kind of image to be printed). In such an arrangement, when the software calibration is effected, the patch is printed on the basis of the patch data based on the image processing conditions (binarizing method) set by the user, and, on the other hand, when the device calibration is effected, the patch is outputted on the basis of the image processing conditions previously set in the printer engine. As a result, the image processing conditions may be differentiated from each other.